K2-33b is an exoplanet, which means that it orbits a star beyond our sun. K2-33b is about 50 percent larger than Neptune – six times the size of the Earth – and is a mere 5 to 10 million years old, making it the youngest fully formed exoplanet ever detected.

Compared to the Earth, which is a “middle-aged” planet at about 4.5 billion years old, “you might think of [K2-33b] as an infant,” Trevor David, a graduate student working with astronomer Lynne Hillenbrand at the California Institute of Technology (Caltech), and lead author of a new study published online Monday in the journal Nature, said in a press release.

The exoplanet was first detected by K2 when the telescope’s camera noticed a regular dimming of the light emitted by K2-33b’s host star, as if the light was blocked by an orbiting planet passing periodically in front of the star.

Mauna Kea's Keck Observatory confirmed that the dimming was caused by a “newborn” exoplanet formed out of the host star’s protoplanetary disk, the thick disk made of gas and dust that surrounds young stars during their planet-formation phase. K2-33b’s host star is currently surrounded by a thin disk of planetary debris, which indicates that it’s nearing the end of this phase.

"Initially, this material may obscure any forming planets, but after a few million years, the dust starts to dissipate," co-author Anne Marie Cody, a postdoctoral program fellow at NASA's Ames Research Center in California, said in a statement. "It is during this time window that we can begin to detect the signatures of youthful planets with K2.”

One of the things that makes K2-33b such a valuable and unique source of potential information is its proximity to its host star: it’s nearly 10 times closer to its star than Mercury is to our sun, with a complete orbit that takes about five days. The only planets with similarly small orbits discovered in the past have been older, which previously led some astronomers to theorize that it might take hundreds of millions of years for a planet to move into an orbit so close to its star.

Currently, there are two main theories to explain K2-33b’s unexpectedly tight orbit. One suggests that the exoplanet migrated there over hundreds of thousands of years in a process called disk migration. The other theory is that K2-33b could have formed “in situ,” or in its current location.

The latter theory would have been considered impossible up until a few years ago, Mr. David said, “but in the past several years, some momentum has grown for in situ formation theories, so the idea is not as wild as it once seemed.”

While K2-33b is a fully formed planet, it still may evolve over time, researchers say. Their next step will be to measure the exoplanet’s mass and determine its density, which will provide insight into whether K2-33b will remain the same size or cool and contract as time goes on.

"This discovery is a remarkable milestone in exoplanet science," Erik Petigura, a postdoctoral scholar in planetary science and a co-author of the paper, said in a press release for Caltech. "The newborn planet K2-33b will help us understand how planets form, which is important for understanding the processes that led to the formation of the earth and eventually the origin of life."